The present invention relates to a radar device for measuring the distance h of the device to a surface, comprising means for transmitting a frequency-modulated wave provided by an oscillator having a frequency control, receiving means for receiving this wave reflected from said surface, a mixing circuit for producing a beat wave between the transmitted and received waves and a processing circuit including a beat wave digitizing circuit and a time-frequency transform means for producing frequency components from which the distance h is determined.
Such a device is used more specifically in the field of radio navigation where it can be used as a radio altimeter. Reference is made to U.S. Pat. No. 4,568,938.
In the radar device disclosed in said patent, a time-frequency transform is effected directly on the beat wave. The frequency components of the lowest frequency where a signal is found provides the distance h. The fact that a time-frequency transform is effected directly on the beat wave leads to a first disadvantage. Actually, the frequency components thus determined are greatly beset with the noise inherent to this type of radar devices and it is thought that the distance h can only be very approximative.
The fact that the lowest frequency component is used to determine the distance h leads to a second disadvantage. This second disadvantage is obvious above all for small distances h and when there is relative motion of the radar device with respect to the surface. In the sequel of this description it will be demonstrated that the spectrum of the beat wave is extended by a function sin x/x where x is a variable depending on h and on the duration of the frequency variation of the oscillator. It follows that the lowest frequency component does not necessarily represent the distance h. In addition, the spectrum of the beat wave can be still further extended by the Doppler effect when there is a relative displacement between the device and the surface.